Light emitting device including semiconductor nanocrystals

ABSTRACT

A light emitting device includes a semiconductor nanocrystal in a layer. The layer can be a non-polymeric layer.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government may have certain rights in this invention pursuantto Grant No. DMR-9808941 from the National Science Foundation.

CLAIM OF PRIORITY

This application claims priority under 35 USC §119(e) to U.S. PatentApplication Ser. No. 60/368,130, filed on Mar. 29, 2002, the entirecontents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to light emitting devices includingsemiconductor nanocrystals.

BACKGROUND

Light-emitting devices can be used, for example, in displays (e.g.,flat-panel displays), screens (e.g., computer screens), and other itemsthat require illumination. Accordingly, the brightness of thelight-emitting device is one important feature of the device. Also, lowoperating voltages and high efficiencies can improve the viability ofproducing emissive devices.

Light-emitting devices can release photons in response to excitation ofan active component of the device. Emission can be stimulated byapplying a voltage across the active component (e.g., anelectroluminescent component) of the device. The electroluminescentcomponent can be a polymer, such as a conjugated organic polymer or apolymer containing electroluminescent moieties or layers of organicmolecules. Typically, the emission can occur by radiative recombinationof an excited charge between layers of a device. The emitted light hasan emission profile that includes a maximum emission wavelength, and anemission intensity, measured in luminance (candelas/square meter (cd/m²)or power flux (W/m²)). The emission profile, and other physicalcharacteristics of the device, can be altered by the electronicstructure (e.g., energy gaps) of the material. For example, thebrightness, range of color, efficiency, operating voltage, and operatinghalf-lives of light-emitting devices can vary based on the structure ofthe device.

SUMMARY

In general, a light emitting device includes a plurality ofsemiconductor nanocrystals. Semiconductor nanocrystals consist of 1-10nm diameter inorganic semiconductor particles decorated with a layer oforganic ligands. These zero-dimensional semiconductor structures showstrong quantum confinement effects that can be harnessed in designingbottom-up chemical approaches to create complex heterostructures withelectronic and optical properties that are tunable with the size of thenanocrystals. The light emitting device can include a layer of a matrix.The matrix can be non-polymeric, for example, a small molecule. Thelight emitting device can include a first electrode proximate to asurface of the layer. A second layer can contact the layer. A secondelectrode can be proximate to the second layer. The semiconductornanocrystal can have a CdSe core and a ZnS shell.

In one aspect, a light emitting device includes a first electrode, alayer including a in a matrix, a first electrode, a second electrodeopposed to the first electrode and a plurality of semiconductornanocrystals disposed between the first electrode and the secondelectrode. The electrodes can be arranged to apply a voltage drop acrossthe layer.

In another aspect, a light emitting device includes a hole transportinglayer proximate to a first electrode arranged to introduce holes in thehole transporting layer, an electron transporting layer proximate to asecond electrode arranged to introduce electrons in the electrontransporting layer, a plurality of semiconductor nanocrystals disposedbetween the first electrode and the second electrode, and a blockinglayer between the first electrode and the second electrode. The blockinglayer can be a hole blocking layer, an electron blocking layer, or ahole and electron blocking layer. The blocking layer can be in contactwith the first electrode or the second electrode.

In another aspect, a method of manufacturing a light emitting deviceincludes depositing a matrix to form a layer, depositing a plurality ofsemiconductor nanocrystals over a first electrode, and placing a secondelectrode over the plurality of semiconductor nanocrystals.

In yet another aspect, a method of generating light includes providing adevice including a first electrode, a second electrode, a layerincluding a matrix, and a plurality of semiconductor nanocrystalsdisposed between the first electrode and the second electrode, andapplying a light-generating potential across the first electrode and thesecond electrode.

The matrix can be non-polymeric. A non-polymeric material can have amolecular weight less than 2,000. The plurality of semiconductornanocrystals can be a substantially monodisperse population ofsemiconductor nanocrystals, or more than one population.

The layer can be a hole transporting layer. The device can include anelectron transporting layer, an electron blocking layer, a hole blockinglayer, a hole and electron blocking layer, or combinations thereofbetween the first electrode and the hole transporting layer.

The light emitting device can have an external quantum efficiency ofgreater than 0.1%, greater than 0.2%, greater than 0.3%, greater than0.4%, or greater than 0.6% at a current density of 7 mA/cm², or greaterthan 1.0% at a current density of 1 mA/cm². The light emitting devicecan have a device luminance of greater than 1000 cd/m², or between 1200and 1500 cd/m² at a current density of 125 mA/cm². The device can have aluminescence efficiency of 1.2 cd/A. For example, the device can have amaximum emission wavelength of 570 nm and can have a full width at halfmaximum of 36 nm. The yield over hundreds of devices is greater than90%.

Narrow size distribution, high quality nanocrystals with highfluorescence efficiency are first prepared using previously establishedliterature procedures and used as the building blocks. See, C. B. Murrayet al., J. Amer. Chem. Soc. 1993, 115, 8706, B. O. Dabbousi et al., J.Phys. Chem. B 1997, 101, 9463, each of which is incorporated byreference in its entirety. The organic, surface-passivating ligands arethen exchanged to stabilize the nanocrystals in polar solvents and inthe matrix.

The layer can include greater than 0.001%, greater than 0.01%, greaterthan 0.1%, greater than 1%, greater than 5%, greater than 10%, greaterthan 50%, or greater than 90% by volume semiconductor nanocrystals. Alayer can be a monolayer of semiconductor nanocrystals. Each of theplurality of semiconductor nanocrystals includes a first semiconductormaterial. Each first semiconductor material can be overcoated with asame or different second semiconductor material. Each firstsemiconductor film has a first band gap and each second semiconductormaterial has a second band gap. The second band gap can be larger thanthe first band gap. Each nanocrystal can have a diameter of less thanabout 10 nanometers. The plurality of nanocrystals can have amonodisperse distribution of sizes.

There has been an increasing interest in light emitting devices based onorganic materials, motivated in part by a wide range of applications,including flat panel displays. Advantageously, the emission frequenciesof the light emitting devices including nanocrystals can be tunedwithout changing the structure of the device. Colloidal semiconductornanocrystals exhibit size dependent optical properties due to strongquantum confinement effects. The emission colors of CdSe nanocrystalscan vary from blue to red simply by changing their size. Their emissionspectra can also show narrow Gaussian linewidths, which can be less than30 nm. The addition of a shell of ZnS around CdSe cores results inovercoated nanocrystals that are highly stable, luminescent and can bedispersed in a range of organic environments. These features enhance thefeasibility of using nanocrystals as the emitting material in lightemitting devices.

Electrically pumped light emitting devices including nanocrystals as theelectroluminescent material can be prepared in a controlled fabricationprocess environment which can enhance device lifetime. The tunability ofthe emission frequencies of the nanocrystals can allow multi-color flatpanel displays to be prepared using them.

Other features, objects, and advantages of the invention will beapparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic drawing depicting a light-emitting device.

FIGS. 2A-G are schematic drawings depicting light-emitting devicestructures.

FIG. 3 is a graph depicting an electroluminescence spectrum of ananocrystal light emitting device.

FIG. 4A-B are a graphs depicting external quantum efficiency andcurrent-voltage plots for nanocrystal light emitting devices.

FIG. 5 is a graph depicting photoluminescence spectra of singlecomponent films, a host:guestN,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine:nanocrystalfilm, and the device in the inset of FIG. 2.

FIG. 6 is a graph depicting a proposed energy level diagram of thedevice of FIG. 2.

DETAILED DESCRIPTION

A light emitting device can include two layers separating two electrodesof the device. The material of one layer can be chosen based on thematerial's ability to transport holes, or the hole transporting layer(HTL). The material of the other layer can be chosen based on thematerial's ability to transport electrons, or the electron transportinglayer (ETL). The electron transporting layer typically includes anelectroluminescent layer. When a voltage is applied, one electrodeinjects holes (positive charge carriers) into the hole transportinglayer, while the other electrode injects electrons into the electrontransporting layer. The injected holes and electrons each migrate towardthe oppositely charged electrode. When an electron and hole localize onthe same molecule, an exciton is formed, which can recombine to emitlight.

A light emitting device can have a structure such as shown in FIG. 1, inwhich a first electrode 2, a first layer 3 in contact with the electrode2, a second layer 4 in contact with the layer 3, and a second electrode5 in contact with the second layer 4. First layer 3 can be a holetransporting layer and second layer 4 can be an electron transportinglayer. At least one layer can be non-polymeric. Alternatively, aseparate emissive layer (not shown in FIG. 1) can be included betweenthe hole transporting layer and the electron transporting layer. One ofthe electrodes of the structure is in contact with a substrate 1. Eachelectrode can contact a power supply to provide a voltage across thestructure. Electroluminescence can be produced by the emissive layer ofthe heterostructure when a voltage of proper polarity is applied acrossthe heterostructure. First layer 3 can include a plurality ofsemiconductor nanocrystals, for example, a substantially monodispersepopulation of nanocrystals. Alternatively, a separate emissive layer caninclude the plurality of nanocrystals. A layer that includesnanocrystals can be a monolayer of nanocrystals.

The substrate can be opaque or transparent. The substrate can be rigidor flexible. The substrate can be plastic, metal or glass. The firstelectrode can be, for example, a high work function hole-injectingconductor, such as an indium tin oxide (ITO) layer. Other firstelectrode materials can include gallium indium tin oxide, zinc indiumtin oxide, titanium nitride, or polyaniline. The second electrode canbe, for example, a low work function (e.g., less than 4.0 eV),electron-injecting, metal, such as Al, Ba, Yb, Ca, a lithium-aluminumalloy (Li:Al), or a magnesium-silver alloy (Mg:Ag). The secondelectrode, such as Mg:Ag, can be covered with an opaque protective metallayer, for example, a layer of Ag for protecting the cathode layer fromatmospheric oxidation, or a relatively thin layer of substantiallytransparent ITO. The first electrode can have a thickness of about 500Angstroms to 4000 Angstroms. The first layer can have a thickness ofabout 50 Angstroms to about 1000 Angstroms. The separate emissive layercan have a thickness of about 50 Angstroms to about 200 Angstroms. Thesecond layer can have a thickness of about 50 Angstroms to about 1000Angstroms. The second electrode can have a thickness of about 50Angstroms to greater than about 1000 Angstroms.

The electron transporting layer (ETL) can be a molecular matrix. Themolecular matrix can be non-polymeric. The molecular matrix can includea small molecule, for example, a metal complex. For example, the metalcomplex can be a metal complex of 8-hydroxyquinoline. The metal complexof 8-hydroxyquinoline can be an aluminum, gallium, indium, zinc ormagnesium complex, for example, aluminum tris(8-hydroxyquinoline)(Alq₃). Other classes of materials in the ETL can include metalthioxinoid compounds, oxadiazole metal chelates, triazoles,sexithiophene derivatives, pyrazine, and styrylanthracene derivatives.The hole transporting layer can include an organic chromophore. Theorganic chromophore can be a phenyl amine, such as, for example,N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine(TPD). The HTL can include a polyaniline, a polypyrrole, apoly(phenylene vinylene), copper phthalocyanine, an aromatic tertiaryamine or polynuclear aromatic tertiary amine, a4,4′-bis(9-carbazolyl)-1,1′-biphenyl compound, or anN,N,N′,N′-tetraarylbenzidine.

The layers can be deposited on a surface of one of the electrodes byspin coating, dip coating, vapor deposition, or other thin filmdeposition methods. The second electrode can be sandwiched, sputtered,or evaporated onto the exposed surface of the solid layer. One or bothof the electrodes can be patterned. The electrodes of the device can beconnected to a voltage source by electrically conductive pathways. Uponapplication of the voltage, light is generated from the device.

When the electron and hole localize on a nanocrystal, emission can occurat an emission wavelength. The emission has a frequency that correspondsto the band gap of the quantum confined semiconductor material. The bandgap is a function of the size of the nanocrystal. Nanocrystals havingsmall diameters can have properties intermediate between molecular andbulk forms of matter. For example, nanocrystals based on semiconductormaterials having small diameters can exhibit quantum confinement of boththe electron and hole in all three dimensions, which leads to anincrease in the effective band gap of the material with decreasingcrystallite size. Consequently, both the optical absorption and emissionof nanocrystals shift to the blue, or to higher energies, as the size ofthe crystallites decreases.

The emission from the nanocrystal can be a narrow Gaussian emission bandthat can be tuned through the complete wavelength range of theultraviolet, visible, or infrared regions of the spectrum by varying thesize of the nanocrystal, the composition of the nanocrystal, or both.For example, CdSe can be tuned in the visible region and InAs can betuned in the infrared region. The narrow size distribution of apopulation of nanocrystals can result in emission of light in a narrowspectral range. The population can be monodisperse and can exhibit lessthan a 15% rms deviation in diameter of the nanocrystals, preferablyless than 10%, more preferably less than 5%. Spectral emissions in anarrow range of no greater than about 75 nm, preferably 60 nm, morepreferably 40 nm, and most preferably 30 nm full width at half max(FWHM) can be observed. The breadth of the emission decreases as thedispersity of nanocrystal diameters decreases. Semiconductornanocrystals can have high emission quantum efficiencies such as greaterthan 10%, 20%, 30%, 40%, 50%, 60%, 70%, or 80%.

The semiconductor forming the nanocrystals can include Group II-VIcompounds, Group II-V compounds, Group III-VI compounds, Group III-Vcompounds, Group IV-VI compounds, Group 1-III-VI compounds, GroupII-IV-VI compounds, or Group II-IV-V compounds, for example, ZnS, ZnSe,ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN, GaP,GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe,PbTe, or mixtures thereof.

Methods of preparing monodisperse semiconductor nanocrystals includepyrolysis of organometallic reagents, such as dimethyl cadmium, injectedinto a hot, coordinating solvent. This permits discrete nucleation andresults in the controlled growth of macroscopic quantities ofnanocrystals. Preparation and manipulation of nanocrystals aredescribed, for example, in U.S. Pat. No. 6,322,901, which isincorporated by reference in its entirety. The method of manufacturing ananocrystal is a colloidal growth process. Colloidal growth occurs byrapidly injecting an M donor and an X donor into a hot coordinatingsolvent. The injection produces a nucleus that can be grown in acontrolled manner to form a nanocrystal. The reaction mixture can begently heated to grow and anneal the nanocrystal. Both the average sizeand the size distribution of the nanocrystals in a sample are dependenton the growth temperature. The growth temperature necessary to maintainsteady growth increases with increasing average crystal size. Thenanocrystal is a member of a population of nanocrystals. As a result ofthe discrete nucleation and controlled growth, the population ofnanocrystals obtained has a narrow, monodisperse distribution ofdiameters. The monodisperse distribution of diameters can also bereferred to as a size. The process of controlled growth and annealing ofthe nanocrystals in the coordinating solvent that follows nucleation canalso result in uniform surface derivatization and regular corestructures. As the size distribution sharpens, the temperature can beraised to maintain steady growth. By adding more M donor or X donor, thegrowth period can be shortened.

The M donor can be an inorganic compound, an organometallic compound, orelemental metal. M is cadmium, zinc, magnesium, mercury, aluminum,gallium, indium or thallium. The X donor is a compound capable ofreacting with the M donor to form a material with the general formulaMX. Typically, the X donor is a chalcogenide donor or a pnictide donor,such as a phosphine chalcogenide, a bis(silyl) chalcogenide, dioxygen,an ammonium salt, or a tris(silyl) pnictide. Suitable X donors includedioxygen, bis(trimethylsilyl) selenide ((TMS)₂Se), trialkyl phosphineselenides such as (tri-n-octylphosphine) selenide (TOPSe) or(tri-n-butylphosphine) selenide (TBPSe), trialkyl phosphine telluridessuch as (tri-n-octylphosphine) telluride (TOPTe) orhexapropylphosphorustriamide telluride (HPPTTe),bis(trimethylsilyl)telluride ((TMS)₂Te), bis(trimethylsilyl)sulfide((TMS)₂S), a trialkyl phosphine sulfide such as (tri-n-octylphosphine)sulfide (TOPS), an ammonium salt such as an ammonium halide (e.g.,NH₄Cl), tris(trimethylsilyl) phosphide ((TMS)₃P), tris(trimethylsilyl)arsenide ((TMS)₃As), or tris(trimethylsilyl) antimonide ((TMS)₃Sb). Incertain embodiments, the M donor and the X donor can be moieties withinthe same molecule.

A coordinating solvent can help control the growth of the nanocrystal.The coordinating solvent is a compound having a donor lone pair that,for example, has a lone electron pair available to coordinate to asurface of the growing nanocrystal. Solvent coordination can stabilizethe growing nanocrystal. Typical coordinating solvents include alkylphosphines, alkyl phosphine oxides, alkyl phosphonic acids, or alkylphosphinic acids, however, other coordinating solvents, such aspyridines, furans, and amines may also be suitable for the nanocrystalproduction. Examples of suitable coordinating solvents include pyridine,tri-n-octyl phosphine (TOP), tri-n-octyl phosphine oxide (TOPO) andtris-hydroxylpropylphosphine (tHPP). Technical grade TOPO can be used.

Size distribution during the growth stage of the reaction can beestimated by monitoring the absorption line widths of the particles.Modification of the reaction temperature in response to changes in theabsorption spectrum of the particles allows the maintenance of a sharpparticle size distribution during growth. Reactants can be added to thenucleation solution during crystal growth to grow larger crystals. Bystopping growth at a particular nanocrystal average diameter andchoosing the proper composition of the semiconducting material, theemission spectra of the nanocrystals can be tuned continuously over thewavelength range of 300 nm to 5 microns, or from 400 nm to 800 nm forCdSe and CdTe. The nanocrystal has a diameter of less than 150 Å. Apopulation of nanocrystals has average diameters in the range of 15 Å to125 Å.

The nanocrystal can be a member of a population of nanocrystals having anarrow size distribution. The nanocrystal can be a sphere, rod, disk, orother shape. The nanocrystal can include a core of a semiconductormaterial. The nanocrystal can include a core having the formula MX,where M is cadmium, zinc, magnesium, mercury, aluminum, gallium, indium,thallium, or mixtures thereof, and X is oxygen, sulfur, selenium,tellurium, nitrogen, phosphorus, arsenic, antimony, or mixtures thereof.

The core can have an overcoating on a surface of the core. Theovercoating can be a semiconductor material having a compositiondifferent from the composition of the core. The overcoat of asemiconductor material on a surface of the nanocrystal can include aGroup II-VI compounds, Group II-V compounds, Group III-VI compounds,Group III-V compounds, Group IV-VI compounds, Group 1-III-VI compounds,Group II-IV-VI compounds, and Group II-IV-V compounds, for example, ZnS,ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN,GaP, GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS,PbSe, PbTe, or mixtures thereof. For example, ZnS, ZnSe or CdSovercoatings can be grown on CdSe or CdTe nanocrystals. An overcoatingprocess is described, for example, in U.S. Pat. No. 6,322,901. Byadjusting the temperature of the reaction mixture during overcoating andmonitoring the absorption spectrum of the core, over coated materialshaving high emission quantum efficiencies and narrow size distributionscan be obtained. The overcoating can be between 1 and 10 monolayersthick.

The particle size distribution can be further refined by size selectiveprecipitation with a poor solvent for the nanocrystals, such asmethanol/butanol as described in U.S. Pat. No. 6,322,901. For example,nanocrystals can be dispersed in a solution of 10% butanol in hexane.Methanol can be added dropwise to this stiffing solution untilopalescence persists. Separation of supernatant and flocculate bycentrifugation produces a precipitate enriched with the largestcrystallites in the sample. This procedure can be repeated until nofurther sharpening of the optical absorption spectrum is noted.Size-selective precipitation can be carried out in a variety ofsolvent/nonsolvent pairs, including pyridine/hexane andchloroform/methanol. The size-selected nanocrystal population can haveno more than a 15% rms deviation from mean diameter, preferably 10% rmsdeviation or less, and more preferably 5% rms deviation or less.

The outer surface of the nanocrystal can include a layer of compoundsderived from the coordinating solvent used during the growth process.The surface can be modified by repeated exposure to an excess of acompeting coordinating group to form an overlayer. For example, adispersion of the capped nanocrystal can be treated with a coordinatingorganic compound, such as pyridine, to produce crystallites whichdisperse readily in pyridine, methanol, and aromatics but no longerdisperse in aliphatic solvents. Such a surface exchange process can becarried out with any compound capable of coordinating to or bonding withthe outer surface of the nanocrystal, including, for example,phosphines, thiols, amines and phosphates. The nanocrystal can beexposed to short chain polymers which exhibit an affinity for thesurface and which terminate in a moiety having an affinity for asuspension or dispersion medium. Such affinity improves the stability ofthe suspension and discourages flocculation of the nanocrystal.Nanocrystal outer layers are described in U.S. Pat. No. 6,251,303, whichis incorporated by reference in its entirety.

More specifically, the coordinating ligand can have the formula:

wherein k is 2, 3 or 5, and n is 1, 2, 3, 4 or 5 such that k-n is notless than zero; X is O, S, S═O, SO₂, Se, Se═O, N, N═O, P, P═O, As, orAs═O; each of Y and L, independently, is aryl, heteroaryl, or a straightor branched C₂₋₁₂ hydrocarbon chain optionally containing at least onedouble bond, at least one triple bond, or at least one double bond andone triple bond, the hydrocarbon chain being optionally substituted withone or more C₁₋₄ alkyl, C₂₋₄ alkenyl, C₂₋₄ alkynyl, C₁₋₄ alkoxy,hydroxyl, halo, amino, nitro, cyano, C₃₋₅ cycloalkyl, 3-5 memberedheterocycloalkyl, aryl, heteroaryl, C₁₋₄ alkylcarbonyloxy, C₁₋₄alkyloxycarbonyl, C₁₋₄ alkylcarbonyl, or formyl and the hydrocarbonchain being optionally interrupted by —O—, —S—, —N(R^(a))—,—N(R^(a))—C(O)—O—, —O—C(O)—N(R^(a))—, —N(R^(a))—C(O)—N(R^(b))—,—O—C(O)—O—, —P(R^(a))—, or —P(O)(R^(a))—; and each of R^(a) and R^(b),independently, is hydrogen, alkyl, alkenyl, alkynyl, alkoxy,hydroxylalkyl, hydroxyl, or haloalkyl.

An aryl group is a substituted or unsubstituted cyclic aromatic group.Examples include phenyl, benzyl, naphthyl, tolyl, anthracyl,nitrophenyl, or halophenyl. A heteroaryl group is an aryl group with oneor more heteroatoms in the ring, for instance furyl, pyridyl, pyrrolyl,phenanthryl.

A suitable coordinating ligand can be purchased commercially or preparedby ordinary synthetic organic techniques, for example, as described inJ. March, Advanced Organic Chemistry, which is incorporated by referencein its entirety.

Layers including nanocrystals can be formed by redispersing the powdersemiconductor nanocrystals described above in a solvent system and dropcasting films of the nanocrystals from the dispersion. The solventsystem for drop casting depends on the chemical character of the outersurface of the nanocrystal, i.e., whether or not the nanocrystal isreadily dispersible in the solvent system. The drop cast films are driedin an inert atmosphere for about 12 to 24 hours before being dried undervacuum. Typically, the films are formed on substrates.

Transmission electron microscopy (TEM) can provide information about thesize, shape, and distribution of the nanocrystal population. Powderx-ray diffraction (XRD) patterns can provide the most completeinformation regarding the type and quality of the crystal structure ofthe nanocrystals. Estimates of size are also possible since particlediameter is inversely related, via the X-ray coherence length, to thepeak width. For example, the diameter of the nanocrystal can be measureddirectly by transmission electron microscopy or estimated from x-raydiffraction data using, for example, the Scherrer equation. It also canbe estimated from the UV/Vis absorption spectrum.

For example, nanocrystals can be dispersed in aN,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine(TPD) matrix (a hole transport organic layer; HTL) to yield an efficientlight emitting device. A dispersion including the nanocrystals in a HTLnot only circumvents the relatively poor conduction observed innanocrystal solids, but can also reduce the number of pinhole shorts inthe nanocrystal layer. The dispersion of nanocrystals can form anemissive molecule layer (EML). TPD is a wide band-gap material that canfacilitate the hole injection into the low lying nanocrystal valenceenergy levels and avoid the reabsorption of the nanocrystal emission.Nanocrystals capped with, for example, TOPO, can accept injection ofholes, electrons, or excitons. TPD and nanocrystals are both dispersedin a suitable solvent (chloroform in this case); the mixed solution isspin-coated on top of precleaned ITO substrates. A layer of aluminumtris(8-hydroxyquinoline) (Alq₃) followed by the metal electrode layersare then deposited via thermal evaporation. The device is grown in acontrolled (oxygen-free and moisture-free) environment, preventing thequenching of luminescent efficiency during the fabrication process. TheTPD is the HTL while the Alq_(a) acts as an electron transport layer(ETL). This separation of function allows placement of the hole/electronrecombination (e.g., exciton) recombination zone. The Alq₃ layerthickness is chosen to separate the hole/electron recombination zonefrom the metal electrode that would otherwise quench the radiativerecombination. Device structures are shown in FIGS. 2A-G. Othermultilayer structures may be used to improve the device performance. Anelectron blocking layer (EBL), a hole blocking layer (HBL) or a hole andelectron blocking layer (eBL), can be introduced in the structure asshown, for example, in FIGS. 2C-G. A blocking layer can include3-(4-biphenyl)-1)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ),3,4,5-triphenyl-1,2,4-triazole,3,5-bis(4-tert-butylphenyl)-4-phenyl-1,2,4-triazole, bathocuproine(BCP), 4,4′,4″-tris{N-(3-methylphenyl)-N-phenylamino}triphenylamine(m-MTDATA), polyethylene dioxythiophene (PEDOT),1,3-bis(5-(4-diphenylamino)phenyl-1,3,4-oxadiazol-2-yl)benzene,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole,1,3-bis[5-(4-(1,1-dimethylethyl)phenyl)-1,3,4-oxadiazol-2-yl]benzene,1,4-bis(5-(4-diphenylamino)phenyl-1,3,4-oxadiazol-2-yl)benzene, or1,3,5-tris[5-(4-(1,1-dimethylethyl)phenyl)-1,3,4-oxadiazol-2-yl]benzene.For example, a HBL of BCP can be deposited on top of the TPD-nanocrystallayer followed by the Alq_(a) and the metal electrode layers in order toblock any hole carriers going into the Alq_(a) layer. This can prohibitany Alq₃ emission and improve the spectral purity.

Two pathways to nanocrystal emission can be realized. Charge can bedirectly injected into the nanocrystals from the host matrix, resultingin exciton formation and photon emission. Alternatively, an exciton maybe created in the organic host matrix and transferred via Forster orDexter energy transfer directly to the nanocrystal or via a ligandcovalently attached to the nanocrystal, which then emits at itscharacteristic frequency. Once electrons and holes are successfullyinjected into the nanocrystal, the electron-hole pair (exciton) canrecombine radioactively and emit a photon. FIG. 3 shows (a) the emissionspectrum and (b) the current dependent efficiency profile obtained fromthe nanocrystal light emitting devices described above. The spectrum isdominated by the nanocrystal emission mixed with a relatively small Alq₃emission. Reproducible external efficiencies of about 1.0% can beobtained. The turn-on voltage is about 6V for a current density of 0.1mA/cm².

The performance of organic light emitting devices can be improved byincreasing their efficiency, narrowing or broadening their emissionspectra, or polarizing their emission. See, for example, Bulović et al.,Semiconductors and Semimetals 64, 255 (2000), Adachi et al., Appl. Phys.Lett. 78, 1622 (2001), Yamasaki et al., Appl. Phys. Lett. 76, 1243(2000), Dirr et al., Jpn. J. Appl. Phys. 37, 1457 (1998), and D'Andradeet al., MRS Fall Meeting, BB6.2 (2001), each of which is incorporatedherein by reference in its entirety. Nanocrystals can be included inefficient hybrid organic/inorganic light emitting devices.

Nanocrystals of CdSe coated with a ZnS passivation layer can havephotoluminescence quantum efficiencies of as high as 50%, matching thatof the best organic lumophores. See, for example, Hines et al., J. Phys.Chem. 100, 468 (1996), which is incorporated by reference in itsentirety. By changing the diameter of the CdSe core from 23 to 55 Å, theluminescence wavelength can be precisely tuned from 470 nm to 640 nmwith a typical spectral full width at half of maximum (FWHM) of lessthan 40 nm. See, for example, Dabbousi et al., J. Phys. Chem. 101, 9463(1997), which is incorporated by reference in its entirety. The narrowFWHM of nanocrystals can result in saturated color emission. This canlead to efficient nanocrystal-light emitting devices even in the red andblue parts of the spectrum, since in nanocrystal emitting devices nophotons are lost to infrared and UV emission. The broadly tunable,saturated color emission over the entire visible spectrum of a singlematerial system is unmatched by any class of organic chromophores. Amonodisperse population of nanocrystals will emit light spanning anarrow range of wavelengths. A device including more than one size ofnanocrystal can emit light in more than one narrow range of wavelengths.The color of emitted light perceived by a viewer can be controlled byselecting appropriate combinations of nanocrystal sizes and materials inthe device. Furthermore, environmental stability of covalently bondedinorganic nanocrystals suggests that device lifetimes of hybridorganic/inorganic light emitting devices should match or exceed that ofall-organic light emitting devices, when nanocrystals are used asluminescent centers. The degeneracy of the band edge energy levels ofnanocrystals facilitates capture and radiative recombination of allpossible excitons, whether generated by direct charge injection orenergy transfer. The maximum theoretical nanocrystal-light emittingdevice efficiencies are therefore comparable to the unity efficiency ofphosphorescent organic light emitting devices. The excited statelifetime (τ) of the nanocrystal is much shorter (τ≈10 ns) than a typicalphosphor (τ>0.5 μs), enabling nanocrystal-light emitting devices tooperate efficiently even at high current density.

Devices can be prepared that emit visible or infrared light. The sizeand material of a semiconductor nanocrystal can be selected such thatthe nanocrystal emits visible or infrared light of a selectedwavelength. The wavelength can be between 300 and 2,500 nm or greater,for instance between 300 and 400 nm, between 400 and 700 nm, between 700and 1100 nm, between 1100 and 2500 nm, or greater than 2500 nm. Forexample, a device including PbSe nanocrystals can emit infrared light ofwavelengths between 1200 and 2500 nm, for example between 1300 and 1600nm. More specifically, a device including an HTL of TPD, ˜4 nm diameterPbSe nanocrystals with a capping layer of oleic acid, and an ETL of Alq₃can emit light with a wavelength of 1550 nm.

Electrically pumped molecular organic structures including semiconductornanocrystals can form organic light emitting devices that exhibitefficient electroluminescence. A drawing of a light emitting device isshown in FIG. 2A, along with A schematic drawing of a core-shell typenanocrystal passivated with trioctylphosphine oxide (TOPO) caps is shownin the inset of FIG. 3. The nanocrystal solutions, which can be preparedby the synthetic technique of Murray, et al., J. Am. Chem. Soc. 115,8706 (1993), which is incorporated by reference in its entirety, haveemission spectra that peak at 562 nm, with an absorption maximum at 548nm. The CdSe core diameter is approximately 38 Å, and is overcoated with1.5 monolayers of ZnS. The solution photoluminescence efficiency of thenanocrystals used in this device preparation is 30%. By increasing theovercoating thickness from 1 to 6 monolayers, the efficiency ofelectroluminescence of a 48 Å diameter CdSe core nanocrystal increasesby nearly a factor of two, which is greater than the increase inefficiency of photoluminescence of the solutions of the nanocrystals.Thus the transfer of excitons into the emissive semiconductornanocrystals seems to have increased in tandem with the increasedefficiency of emission once the nanocrystal is excited. This resultsuggests that the dominant nanocrystal excitation mechanism in thesedevices is exciton energy transfer from neighboring organic molecules.The nanocrystals are mixed in various concentrations into a chloroformsolution ofN,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine(TPD), which is then spin-cast onto clean, ITO coated glass substrates,resulting in a 40 nm thick film. A 40 nm thick film oftris(8-hydroxyquinoline) aluminum (Alq₃) is then thermally evaporatedonto the TPD:nanocrystal layer, and capped by a 1 mm diameter, 75 nmthick (10:1 by mass) Mg:Ag cathode with a 50 nm Ag cap. The spin-castingand device manipulation during growth is performed in a dry nitrogenenvironment, with moisture and oxygen content of less than 5 ppm. Allmeasurements are done in air.

The choice of organic host for the nanocrystals is limited by materialdeposition methods. CdSe nanocrystals are typically arranged into thinfilms by spin-casting from solution. While spin-casting is possible formolecular organics, and typical for polymer organics, it limits theavailable organic matrix materials to those that are highly soluble insolvents such as toluene, hexanes and chloroform, which are thepreferred solvents for the TOPO capped nanocrystal colloids. In order tohave a large range of possible solution mixtures and film thicknesses,it is necessary to have organic solubility in the range of 10 mg/mL.Such is the case for TPD in chloroform. TPD has the added advantage ofbeing a blue emitting material, which can facilitate access to theentire visible spectrum by doping different sized nanocrystals into thisorganic matrix. A typical nanocrystal-light emitting device emission isshown in FIG. 3. The dashed lines show the decomposition of the spectruminto an Alq₃ component and a nanocrystal component. Insets show theschematics of the device structure and a core-shell type nanocrystal.The spectral peak at 562 nm is due to the nanocrystals, and the broadershoulder centered at 530 nm, attributable to Alq₃ emission. The dashedlines show the decomposition of the electroluminescence spectrum intoAlq₃ and nanocrystal contributions. The integrated intensity ofnanocrystal emission was 60% of the total device luminescence.

The external quantum efficiency of nanocrystal-light emitting devices asa function of current is shown in FIG. 4. An efficiency of 0.45% isobtained at 7 mA/cm² and 10.5 V. The quantum efficiency was above 0.5%for a broad range of device luminances (from 5 to 1900 cd/m²). Thequantum efficiency was 0.61% at 21 mA/cm². At 125 mA/cm², the lightemitting device luminance was 1900 cd/m², which corresponds to aluminescence efficiency of 1.5 cd/A. This is a 25 fold improvement overthe best previously reported nanocrystal-light emitting device result.See, for example, Schlamp, et al., J. Appl. Phys. 82, 5837 (1997). Thepeak external quantum efficiency was above 1.0% between 0.1 and 1.0mA/cm². Device yields over hundreds of devices are greater than 90%,indicating a robust material system.

The spectrum and efficiency of nanocrystal-light emitting devicesstrongly depends on nanocrystal concentration in the TPD matrix. For lowconcentrations of nanocrystals the device behavior is similar to anundoped structure, and at extremely high nanocrystal concentrations amorphology change in the nanocrystal doped layer is observed that leadsto poor device performance and low yields. The thickness of theTPD:nanocrystal layer also plays a critical role in determining thedevice properties. With a thick TPD:nanocrystal layer, the Alq₃ emissionis completely suppressed at the expense of lower quantum efficiency andhigher turn-on voltage of the device. Thinning this layer leads to anexcess of hole injection, and thus enhanced Alq₃ emission. Analternative method to eliminating the Alq₃ emission without sacrificingefficiency is to use a hole and electron blocking layer such as atriazole between the Alq₃ and TPD:nanocrystal layers. The device showsthe spectral purity that one would expect, with 90% of the emissionbeing due to the nanocrystals. The peak external quantum efficiency is1.0% in such a device, which is consistent with two thirds of theemission of the 0.61% efficient devices being due to nanocrystals.

The observed spectra also show a minimal dependence on current density.Deep trap emission from the nanocrystals is always present as a weakelectroluminescence tail red-shifted from the main emission peak, but itsaturates at very low currents (<1 mA/cm²). This deep trap emission isenhanced when incorporating core only nanocrystals, rather thancore-shell type nanocrystals. With the less stable nanocrystals, thedeep trap emission saturates at much higher current densities (˜100mA/cm²), resulting in light emitting devices with significant emissionin the infrared. For optimum visible light emitting device performancethe overcoated nanocrystals can be used.

Absorption and photoluminescence measurements of thin films, andelectroluminescence from device structures were analyzed. FIG. 5 showsthin film absorption and photoluminescence of neat films of Alq₃, TPD,and nanocrystals, along with a nanocrystal doped TPD film(TPD:nanocrystal) spun from the same solution that was used in thedevice shown in FIG. 2A. Absorption measurements indicated thatnanocrystals make up only 5% by volume of the 400 Å films. Thiscorresponds to a layer that is 20 Å thick, which is not possible sincethe nanocrystals themselves are 50 Å in diameter including theovercoating and organic caps. Thus, the nanocrystals may not be arrangedinto a complete layer, and can play a limited role in conduction, evenif the nanocrystals completely phase segregate from the TPD during thespinning process. A device with a similar structure to that shown inFIG. 2A, but with an additional 50 Å of TPD deposited by thermalevaporation between the spun layer and the Alq₃ was prepared. In asimple TPD/Alq₃ device, no emission was observed from the TPD.Therefore, it appears that all of the excitons were created within oneForster energy transfer radius (˜40 Å) of the Alq₃ interface. By addingthis 50 Å TPD layer, substantially all of the excitons can be created onorganic sites (both TPD and Alq₃ are possible sites). The emissionspectrum of such a device clearly shows that the nanocrystals still emit(35% of total emission is due to nanocrystals in such a device). Thereis exciton energy transfer from TPD to nanocrystals in this device. Itis also possible that excitons can be created directly on thenanocrystals in the other device structure. These two processes cancompete in the different device structures. Photoluminescence spectraare consistent with energy transfer occurring because if it does not,less nanocrystal emission would take place from the nanocrystal:TPDfilms. The enhancement in nanocrystal emission is consistent with aForster energy transfer radius of 30 Å for nanocrystals that are 10%quantum efficient in solid state. This solid state quantum efficiency ofwas determined for a neat film of nanocrystals relative to a neat filmof TPD. Variation of the ZnS overcoating thickness can also be made.

FIG. 6 shows a proposed energy level diagram for the device of FIG. 2B.Where possible, values are taken from ultraviolet photoelectronspectroscopy (UPS) measurements. See, for example, Hill et al., J. Appl.Phys. 86, 4515 (1999), which is incorporated by reference in itsentirety. Nanocrystal levels shown are from calculated values. Electronscan be injected from the Mg cathode into the Alq₃ and are transported tothe heterojunction. Similarly, holes can be injected from the ITOcontact primarily into the TPD host matrix, and are transported towardsthe junction. The relative energy alignment of the lowest unoccupiedmolecular orbital (LUMO) levels of Alq₃ and the nanocrystals results inelectrons trapped at the nanocrystals that are located near thisheterojunction. For these charged nanocrystals the barrier to holeinjection from the TPD is greatly reduced. Upon acceptance of holes fromTPD, excitons form on the nanocrystals, and can subsequently recombineradiatively. The spectrum in FIG. 3 indicates that a fraction ofexcitons are formed on the Alq₃ molecules, contributing to the emissionof green light. However, TPD electroluminescence was not observed inthis device structure, indicating that excitons that are formed on TPDeither undergo energy transfer to Alq₃, or recombine nonradiatively.

The charge trapping mechanism allows for the creation of excitons on thenanocrystals which can exist in any of the eight-fold degenerate excitonstates, all of which may recombine to emit a photon. See, for example,Kuno et al., J. Chem. Phys. 106, 9869 (1997), which is incorporated byreference in its entirety. This is in direct contrast to organicfluorescent lumophores where only one in four electrically generatedexcitons can recombine radiatively. See, for example, Baldo et al.,Nature, 395, 151 (1998), which is incorporated by reference in itsentirety. However, there are other inherent limits to the quantumefficiency of any device utilizing nanocrystals as the emitting centers.Besides the unoptimized initial nanocrystal photoluminescence efficiency(in the devices η≈30%), it has previously been reported that an excitonlocated on a charged nanocrystal is not likely to radiatively recombine.See, for example, Shimazu et al., Phys. Rev. B, 63, 205316-1 (2001),which is incorporated by reference in its entirety. Following an Augerrecombination process, the energy of the exciton is given to the secondexcited electron on the nanocrystal, which could lead to the ejection ofthe second electron from the nanocrystal or its non-radiativerecombination. To achieve high external quantum efficiencies it istherefore necessary to optimize charge injection balance innanocrystal-light emitting devices, or to eliminate charge injectionexcitons as a possibility.

The fundamental limits of nanocrystal-light emitting device performancecan be significantly different than those of organic light emittingdevices. The nanocrystal-light emitting devices have an emission FWHM of31 nm. In contrast, typical molecular organic light emitting deviceshave a FWHM of between 60 and 100 nm, although emission of some polymersand phosphorescent molecules was shown to be as narrow as 26 nm FWHM.See, for example, Liu et al., Appl. Phys. Lett. 79, 578 (2001), andKwong et al., Chem. Mat. 11, 3709 (1999), each of which is incorporatedby reference in its entirety. However, in all of these cases thefundamental limit on bandwidth has already been achieved throughmaterials preparation and purification. The vibrational structure ofsterically flexible organics typically generates broad single moleculeemission spectra at room temperature. See, for example, Tamarat et al.,J. Phys. Chem. A 104, 1 (2000), which is incorporated by reference inits entirety. The same is not true of the rigid, covalently bondedinorganic nanocrystal, for which single nanocrystal spectroscopy showsthat the fundamental FWHM linewidth of a nanocrystal at room temperatureis 14 nm. See, for example, Empedocles et al., Phys. Rev. Lett. 77, 3873(1996), which is incorporated by reference in its entirety. It is thecombination of spectral diffusion and size distribution of nanocrystalsin a sample that yields further line broadening. Consequently, the 31 nmlinewidth corresponds to a size distribution of about 10%. It isreasonable to expect that new techniques in nanocrystal preparation andprocessing could lead to nanocrystal-light emitting device line widthsthat are as narrow as 25 nm. This true color saturation would be idealfor many applications where efficient production of narrowband light isdesired. In particular, the creation of a high luminescent efficiencyred light emitting device can require both high external quantumefficiency as well as narrowband emission, to prevent the bulk ofemission from occurring in the infrared. The deep trap emission that istypical of nanocrystals could be problematic in achieving this goal, butthe devices reported here already show less than 1% of their total poweremitted in the infrared. This deep trap emission saturates at very lowcurrent densities. See, for example, Kuno et al., J. Chem. Phys. 106,9869 (1997). The spectral FWHM reported here is already an improvementover conventional organic light emitting devices, and yet thefundamental limit has not been attained.

A high efficiency light emitting device utilizes molecular organic thinfilms as the electrical transport medium and inorganic CdSe(ZnS)nanocrystals as the lumophores. These devices represent atwenty-five-fold improvement in luminescent power efficiency overpreviously reported nanocrystal-light emitting devices. The mechanismfor light emission is shown to be carrier recombination on thenanocrystals. It is clear that the limit of device performance has notyet been reached, both in quantum efficiency and in color saturation.Development of new deposition techniques for generating homogeneouslydispersed films of nanocrystals in organic matrices should make possiblea much wider range of material hybrids, enabling the creation of lightemitters that are technologically competitive with state of the artorganic and inorganic light emitting devices.

Other embodiments are within the scope of the following claims.

1-86. (canceled)
 87. A composition comprising a nanocrystal and acoordinating ligand, wherein the coordinating ligand has the formula

wherein k is 2, 3 or 5, and n is 1, 2, 3, 4 or 5, such that k-n is notless than zero; and each of Y and L, independently, is aryl, heteroaryl,or a straight or branched C₂₋₁₂ hydrocarbon chain optionally containingat least one double bond, at least one triple bond, or at least onedouble bond and one triple bond, the hydrocarbon chain being optionallysubstituted with one or more C₁₋₄ alkyl, C₂₋₄ alkenyl, C₂₋₄ alkynyl,C₁₋₄ alkoxy, hydroxyl, halo, amine, nitro, cyano, C₃₋₅ cycloalkyl, 3-5membered heterocycloalkyl, aryl, heteroaryl, C₁₋₄ alkylcarbonyloxy, C₁₋₄alkyloxycarbonyl, C₁₋₄ alkylcarbonyl, or formyl; and the hydrocarbonchain being optionally interrupted by —O—, —S—, —N(R^(a))—,—N(R^(b))—C(O)—O—, —O—C(O)—N(R^(a))—, —N(R^(a))—C(O)—N(R^(b))—,—O—C(O)—O—, —P(R^(a))—, or —P(O)(R^(a))—, wherein each of R^(a) andR^(b), independently, is hydrogen, alkyl, alkenyl, alkynyl, alkoxy,hydroxylalkyl, hydroxyl, or haloalkyl.
 88. The composition of claim 87,wherein X is O, S, S═O, SO₂, Se, Se═O, N, N═O, P, P═O, As, or As═O. 89.The composition of claim 87, wherein the nanocrystals comprise at leastone semiconductor material selected from the group consisting of Zns,ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN,GaP, GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS,PbSe, and PbTe.
 90. The composition of claim 89, wherein thenanocrystals further comprise an overcoating, the overcoating comprisingat least one semiconductor material selected from the group consistingof ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs,AlSb, GaN, GaP, GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs,TlSb, PbS, PbSe, and PbTe.
 91. The composition of claim 87, wherein thenanocrystal comprises a CdSe core and a ZnS shell.
 92. The compositionof claim 87, wherein at least a portion of the coordinating ligand hasan affinity for the surface of the nanocrystal.
 93. The composition ofclaim 87, wherein at least a portion of the coordinating ligand has anaffinity for a dispersion medium.
 94. The composition of claim 87,wherein each of Y or L, independently, comprises a straight or branchedC₂₋₁₂ hydrocarbon chain substituted with one or more amines.
 95. Thecomposition of claim 87, wherein each of Y or L, independently,comprises a straight or branched C₂₋₁₂ hydrocarbon chain substitutedwith multiple amines.
 96. The composition of claim 95, wherein the oneor more amine is attached to the surface of the nanocrystal.
 97. Thecomposition of claim 87, wherein each of Y or L, independently,comprises a straight or branched C₂₋₁₂ hydrocarbon chain substitutedwith one or more hydroxyl.
 98. The composition of claim 87, wherein thecoordinating ligand is attached to the surface of the nanocrystal. 99.The composition of claim 87, wherein the coordinating ligand iscovalently bonded to the surface of the nanocrystal.
 100. Thecomposition of claim 87, wherein the coordinating ligand forms apassivation layer on the surface of the nanocrystal.
 101. A lightemitting device comprising the composition of claim
 87. 102. A method ofstabilizing one or more nanocrystals, comprising: coating the one ormore nanocrystals with a coordinating ligand, wherein the coordinatingligand has the formula

wherein k is 2, 3 or 5, and n is 1, 2, 3, 4 or 5, such that k-n is notless than zero; and each of Y and L, independently, is aryl, heteroaryl,or a straight or branched C₂₋₁₂ hydrocarbon chain optionally containingat least one double bond, at least one triple bond, or at least onedouble bond and one triple bond, the hydrocarbon chain being optionallysubstituted with one or more C₁₋₄ alkyl, C₂₋₄ alkenyl, C₂₋₄ alkynyl,C₁₋₄ alkoxy, hydroxyl, halo, amine, nitro, cyano, C₃₋₅ cycloalkyl, 3-5membered heterocycloalkyl, aryl, heteroaryl, C₁₋₄ alkylcarbonyloxy, C₁₋₄alkyloxycarbonyl, C₁₋₄ alkylcarbonyl, or formyl; and the hydrocarbonchain being optionally interrupted by —O—, —S—, —N(R^(a))—,—N(R^(b))—C(O)—O—, —O—C(O)—N(R^(a))—, —N(R^(a))—C(O)—N(R^(b))—,—O—C(O)—O—, —P(R^(a))—, or —P(O)(R^(a))—, wherein each of R^(a) andR^(b), independently, is hydrogen, alkyl, alkenyl, alkynyl, alkoxy,hydroxylalkyl, hydroxyl, or haloalkyl.
 103. The method of claim 102,wherein X is O, S, S═O, SO₂, Se, Se═O, N, N═O, P, P═O, As, or As═O. 104.The method of claim 102, wherein each of Y or L, independently,comprises a straight or branched C₂₋₁₂ hydrocarbon chain substitutedwith one or more amines.
 105. The method of claim 102, wherein each of Yor L, independently, comprises a straight or branched C₂₋₁₂ hydrocarbonchain substituted with multiple amines.
 106. The method of claim 104,wherein the one or more amine is attached to the surface of thenanocrystal.
 107. The method of claim 102, wherein each of Y or L,independently, comprises a straight or branched C₂₋₁₂ hydrocarbon chainsubstituted with one or more hydroxyl.